The present invention relates to the bulk growth of large, high-quality, silicon carbide crystals for electronic and related applications.
Silicon carbide (SiC) is a compound of significant interest as a material for both substrates and active layers for high voltage and high frequency semiconductor devices as well as being of interest in the manufacture and structure of certain types of light emitting diodes.
From an electronic standpoint, silicon carbide has a number of theoretical and practical advantages that make its use desirable in microelectronic devices. Silicon carbide has a wide band gap, a high critical breakdown field (approximately two mega-volts per centimeter), and a high thermal conductivity (about five watts per centimeter-Kelvin). Silicon carbide is also physically very hard. Silicon carbide has a high electron drift velocity, excellent thermal stability, and excellent radiation resistance or “hardness.” These advantages have been recognized and described thoroughly in the patent and non-patent literature.
As is the case with other semiconductor materials, silicon carbide can be grown as “bulk” crystals or as epitaxial layers. Bulk growth generally (although not necessarily exclusively) refers to growth techniques that produce larger crystals for use as substrates and related purposes. Bulk growth techniques, although not necessarily “fast” in an absolute sense, generally proceed at a rate sufficient to make the techniques economically worthwhile and the resulting bulk crystals economically obtainable. Bulk crystal growth typically refers to growth from a seed, but as used herein can also refer to the growth of layers which are sufficiently thick to share the functional characteristics of bulk grown crystals.
By way of comparison, epitaxial growth is typically used to produce smaller portions—most often layers—of a semiconductor material with high purity, high crystal quality, and specific doping parameters. Relatively speaking, epitaxial growth generally proceeds more slowly than bulk growth, but produces a higher quality crystal. Furthermore, because epitaxial layers can serve their purpose even when relatively thin, the longer time required to grow them is acceptable in exchange for their higher quality.
Bulk growth of silicon carbide is typically carried out by one of two methods: sublimation from a source powder or high temperature chemical vapor deposition (HTCVD).
The HTCVD technique uses a seed crystal, but instead of a silicon carbide source powder, silicon containing species (typically silane) and carbon containing species (typically propane) are introduced as gases.
The HTCVD technique can produce high purity, highly uniform material with controlled electronic characteristics. Nevertheless, longer (larger) crystals are hard to obtain because the growth efficiency is relatively low and parasitic reactions compete with the desired silicon carbide growth. Additionally, silane tends to decompose in significant amounts at relatively low temperatures (in some cases below 400° C.) as compared to those needed for bulk SiC growth (e.g. about 2000° C.). HTCVD suffers from other disadvantages including the tendency of the reaction products to deposit everywhere—i.e. throughout the deposition chamber as well as on the desired surface—which wastes material and requires the deposition apparatus to be cleaned frequently.
Furthermore, the displacement reactions characteristic of HTCVD typically generate hydrogen as a reaction product. In turn, hydrogen will tend to etch silicon carbide at HTCVD temperatures.
Sublimation, also referred to as physical vapor transport (PVT), is usually carried out in the presence of a seed. In this technique, a seed crystal of silicon carbide and a silicon carbide source powder are both placed into a crucible (typically formed of graphite). The crucible is then heated in a manner that creates a temperature gradient between the source powder and the seed, and with the powder generally being warmer than the seed. At appropriate temperatures (i.e. at least about 1900-2000° C.), silicon carbide source powder will sublime to form gaseous species (dominated by Si, Si2C and SiC2). The temperature gradient encourages the species to migrate to the seed, which is typically maintain about 100-200° C. cooler than the source powder. The migrating species condense on the seed crystal providing the desired crystal growth.
Although relatively well understood and well-established (e.g., commonly assigned U.S. Pat. No. 4,866,005) the static use of source powder in a closed crucible can limit the quality of the crystal eventually produced.
In this regard, it will be understood by those familiar with electronic devices and semiconductor materials that the term “quality,” is applied in a relative sense. Sublimation produces very high-quality silicon carbide crystals for many purposes. Nevertheless, when SiC devices are used at extremely high power—which represents one of SiC's advantages—even a small number of defects can degrade performance noticeably or even lead to device breakdown. Thus, increasing the quality of silicon carbide bulk crystals always remains of interest.
As one particular problem, and because of the thermodynamic differences between silicon and carbon, silicon carbide tends to sublime in a non-stoichiometric fashion. Although the mechanism is not totally understood, the silicon content of the source powder tends to deplete more quickly than the carbon content. This produces a carbon-rich source powder, a characteristic referred to as “powder graphitization.” Even source material that is intentionally made or selected to be silicon-rich becomes graphitized over time.
In turn, powder graphitization causes the ratio of the vaporized silicon and carbon species to change during any given growth run. Such changes can produce undesired changes in the growing crystal. For example, higher silicon-to-carbon ratios tend to produce the 3C polytype of silicon carbide even when the 6H polytype is being used as the seed.
As another potential factor the composition of transported gases that produces the best initial nucleation on the seed crystal may be different from the composition that produces the best bulk growth (and vice versa). Thus, in the conventional physical vapor transport (sublimation) systems, neither nucleation nor bulk growth may be optimized. Instead, both may be compromised based upon the fixed starting material.
Stated differently, in conventional physical vapor transport growth techniques, the relevant system is loaded with a starting material and then heated to drive the sublimation growth of the resulting crystal. The application of heat, however, is typically the only step that can be manipulated during the growth process; i.e., the starting materials are locked in and cannot be modified as growth proceeds.
Other problems exist. For example, where nitrogen is used as a dopant to create n-type material, the normal and expected process is for the nitrogen dopant atoms to replace carbon atoms in the crystal structure. Changing the ratio of silicon-to-carbon, however, causes the nitrogen to compete with a different amount of carbon for a given position in the growing crystal. This, among other factors, can result in intrinsic defects such as silicon vacancies and carbon vacancies. Furthermore, it is generally expected that the formation (or prevention) of micropipes is affected by the silicon to carbon ratio in the vapor phase.
Additionally, these growth issues are of greater concern as the diameter of the growing crystal increases. In this regard, in a commercial context growing larger diameter crystals is usually more efficient than growing smaller diameter crystals. In silicon-based technology, wafers as large as eight inches (200 millimeters) in diameter are commercially available and widely understood. In silicon carbide technology, however, three and four inch wafers (75-100 mm) still remain as a commercial upper limit.
Accordingly, interest continues to exist in improving the techniques for bulk growth of silicon carbide end in correspondingly improving the resulting bulk crystals.
The foregoing and other objects and advantages of the invention and the manner in which the same are accomplished will become clearer based on the followed detailed description taken in conjunction with the accompanying drawings.